The present invention relates to a solid state electronic device for transferring a surface acoustic wave (hereinafter referred to as SAW) having a larger electric power or a solid state electronic device in which a large amplitude SAW exists as a standing wave, and more particularly to a thin film electrode, electric wiring pattern or bonding pad which can be suitably employed in such a solid state electronic device.
In the case where a current of high density as high as 10.sup.5 -10.sup.6 A/cm.sup.2 or more flows through an aluminum (Al) and Al electric wiring pattern of the conventional semiconductor device having an integrated circuit, a phenomenon of migration will occur in the electrode or electric wiring pattern. The migration causes hillocks, voids, etc., which frequently leads to short-circuiting or disconnection of the electrode and electric wiring pattern. The cause thereof is considered to be that Al atoms move at crystal boundaries due to collision of electrons.
Techniques of obviating the migration have been proposed in JP-B-No. 45-1133 (published on Jan. 14, 1970), JP-A-No. 49-22397 (laid-open on Feb. 27, 1974), etc. These conventional techniques use an electrode of an Al alloy in which copper is added to Al.
On the other hand, as disclosed in an article in the Transactions of the Institute of Electronics and Communication Engineers of Japan, Vol. J67C, No. 3, pages 278-285 (March 1984), the same defect as that due to the above migration occurs in an electrode of the solid state electronic device such as a SAW filter for transferring a large electric power, a SAW resonator in which a large amplitude SAW exists as a standing wave. Specifically, in the SAW filter, troubles due to the short-circuiting or disconnection of wirings frequently occurs. Also in the SAW resonator, a remarkable secular change in a resonant frequency disadvantageously occurs.
This article explains the mechanism of producing defects in the SAW device as follows. Distortion in a substrate surface generated by SAW gives rise to an internal stress in an Al electrode (thin film) formed on the substrate surface. The grain boundary in the Al crystal moves at the area where the internal stress exceeds a threshold value and there voids and hillocks are generated. The movement of the grain boundary due to the internal stress is considered to occur through the same mechanism as in the temperature cycle in an integrated circuit which is described in an article in IEEE Transactions on Parts, Hybrids and Packaging, Vol. PHP-7, No. 3, pp. 134-138.
Incidentally, the above JP-B and JP-A propose, as means for obviating the defect of the Al electrode due to the migration, to dope a minute amount (1-4 wt %) of Cu, which has been adopted in a semiconductor integrated circuit.
However, the above conventional technique could not provide desired performances in relation to power handling capability, device loss, mass-productivity, etc. For example, the SAW device for use in a high frequency as high as 800 MHz or so could not assure a life enough to operate using a large electric power, and also provided reduced production yield of wire bondings due to increased hardness of the thin film (electrode).
In co-pending U.S. application (A. Yuhara et al) Ser. No. 263,078 (based on Japanese Patent Application Nos. 61-3428 and 61-46138) filed Oct. 27, 1988 and assigned to the assignee of the present invention which is a continuation of application Ser. No. 2,286 filed Jan. 12, 1987, now abandoned, a SAW device is disclosed in which electrodes are formed on a piezoelectric substrate by sputtering and/or the electrodes contain an additive of Cu, Ti, Zn, Mg, Fe, Ni, Cr, Ga, Ge, Sn, Pd or Ta. The above-mentioned Japanese applications were published as JP-A-No. 62-163408 on July 20, 1987 and as JP-A-No. 62-204611 on Sept. 9, 1987.